By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver
County

Tomato/tobacco hornworms are the largest caterpillars found in this area and can
measure up to 4 inches in length. The prominent "horn" on the rear of both
gives them their name.

The size of these garden pests allow them to quickly defoliate tomatoes, potatoes,
eggplants, and peppers. Occasionally, they may also feed on green fruit. Gardeners are
likely to spot the large areas of damage at the top of a plant before they see the
culprit. Hornworms are often difficult to see because of their protective coloring. Not
much for the heat of direct sunlight, they tend to feed on the interior of the plant
during the day and are more easily spotted when they move to the outside of the plant at
dawn and dusk

The tobacco hornworm larva (Manduca sexta) is generally green with seven
diagonal white lines on the sides and a curved red horn (above). The tomato
hornworms (Manduca quinquemaculata) have eight V-shaped marks on each side and
their horn is straighter and blue-black in color (below). These "hornworms" are
the larvae of hawk or sphinx moths, also known as hummingbird
moths. The tobacco hornworm is the most commonly seen of the two, but both can
be found in this region and may even be present on the same plant.

Tomato Hornworm

The presence of the hornworm may also be noticed because of the large, black droppings
(frass) that accumulate on the ground beneath the affected plants.

Hornworm with frass

Hornworm damage usually begins to occur in midsummer and continues throughout the
remainder of the growing season.

CONTROL:

Handpicking. The large size of hornworms makes it easy to get hold of them.
Once removed from the plant, they can be destroyed by snipping them in half with shears or
dropping them into a bucket of water.

Rototilling. Turning up the soil after harvest will destroy any pupae that may be
there.

Biological. Bacillus thuringensis, or BT (e.g., Dipel, Thuricide), is also
considered very effective, especially on smaller larvae. Spray it as a precaution. Natural
enemies, such as the parasitic wasp that lays its eggs on the hornworm's back, are
common. If found, such worms should be left in the garden so the emerging wasps can
parasitize other hornworms.

Parasitic wasp cocoons on hornworm

Insecticides. Hornworms can be controlled with carbaryl, permethrin, spinosad
insecticides. Read the label carefully before using any insecticide.